All hair cell systems contain efferent feedback projections. In mammals, the olivocochlear efferent fibers originate in the superior olivary complex and project to hair cells and neurons in the cochlea. The olivocochlear (OC) efferent system is composed of medial (M) OC and lateral (L) OC fibers. Calcitonin gene-related peptide (CGRP) is released from LOG fibers onto spiral ganglion fibers and inner hair cells containing the CGRP receptor complex, CGRP knockout animals have a 20% reduction in auditory nerve activity suggesting that efferent fibers containing CGRP are responsible for increasing auditory nerve activity (Maison et al. 2003, J Neurophysiology) and CGRP infused into the perilymph of the cochlea causes a 30% increase in cochlear action potentials (LePrell et al. 2007 ARO abstract #349). The CGRP receptor complex is composed of three proteins (CLR, RAMP1, and RCP). In non-hair cell systems this heterotrimeric CGRP receptor complex has been shown to couple to adenylyl cyclase through GaS to produce an increase in cAMP, a second messenger. Our hypothesis is that LOG fibers release CGRP, which binds to the CGRP receptor complex, activates GaS, and increases cAMP production, which signals to increase auditory nerve activity. In these aims we will determine if: 1) expression of a dominant-negative CGRP receptors can disrupt CGRP-mediated cAMP signaling; and 2) expression of a dominant-negative CGRP receptor can disrupt cochlear function. Successful completion of this project requires cochlear function assessment, co- immunoprecipitation of the CGRP receptor complex, a dominant-negative CGRP receptor, biochemical cAMP measurement in tissue culture cells and FRET-based cAMP measurement on tissue culture cells and microdissected cochleae. Information from this project will elucidate the initial signaling molecular mechanisms used by the CGR-containing LOG efferent pathway, which is hypothesized to contribute to better signal-to-noise detection of sound. [unreadable] [unreadable] [unreadable]